I think of myself as an expert on the biological basis of homosexuality, and concur with the statement above. I published several papers on the topic, including the 1999 Williams Nature paper that I cited below, before my professional interests turned exclusively to neurobiology.

As I noted below, the fraternal birth order effect is the most well-established finding we currently have on the cause of male homosexuality, and it is amenable to an epigenetic intepretation. For example, circumstantial evidence suggests that increasing parity immunizes or innoculates mothers against HY antigens, which *could* underlie the development of male homosexuality. Such a mechanism would probably involve immune cells and cytokines from the mother passing through the placenta and influencing epigenetic marks on the developing fetus' genome. That, in turn, could influence brain and bodily development.

EDIT: Note that the fraternal birth order finding implies that something occurs in utero to affect the psychosexual development of the male fetus in a way that heightens the likelihood that the boy will be gay. We do not know what that something is.

It is considered well-established in this field that prenatal androgen levels sexually differentiate the brain and the body in a male-like direction. Abundant evidence supports this general idea. And since sexual attraction is a sexually dimorphic trait (most men are attracted to women and only women, and vice versa), its reasonable to suppose that something about prenatal androgen could be involved in male homosexuality.